In the recycling of PET bottles, the collected bottle material is first shredded to give so-called flakes which have particle diameters within a range of about 1 to 10 mm. About 70% to 90% of the flakes consist of crystalline PET material from the bottle's wall having a wall thickness of less than about 0.5 mm. About 10% to 30% of the flakes consist of amorphous PET material from the neck and bottom regions of the bottles or from ground preforms having wall thicknesses or particle sizes of about 1 mm or more. In addition, the flakes usually contain impurities, such as e.g. foreign polymers, such as polyvinyl chloride (PVC), polyamide (PA), polyolefins, such as polypropylene (PP) or polyethylene (PE), polylactic acid (PLA) or polystyrene (PS).
These foreign polymers are disturbing in the further thermal processing of the PET material, such as recrystallization or extrusion, as they affect the quality of the PET recyclate. In particular, the presence of major quantities of foreign polymers results in a darkening of the PET recyclate as the foreign polymers are at least partially decomposed under the thermal stress. In addition, in particular PVC contains chloride proportions leading to a decomposition of the polymer chains in the thermal reprocessing of the recyclate. To avoid this, the foreign matters must be largely separated off, i.e. preferably to residues of below about 100 ppm in the PET recyclate.
Conventional separation methods are e.g. methods effective due to differences in the specific material density by which polymers of a lower density than PET, such as PP or PE, can be easily separated off. However, by these methods, e.g. PVC can only be insufficiently separated from PET.
Furthermore, methods of separating foreign polymers from PET material are known which are based on the principle of an increased adhesion tendency of the foreign polymers on surfaces compared to PET. Here, the polymer mixture is heated to a certain temperature above the softening point of the foreign polymers to be separated off, but below the softening temperature of the PET material. Subsequently, the mixture is contacted with a mechanical contact body. By the mechanical adhesion of the foreign polymers to the contact body, they can be separated from the PET. These separation methods are described e.g. in DE4303500A1, DE4004300A1, DE3722777A1, or U.S. Pat. No. 5,236,603. In particular with flakes of PET bottle material having a high proportion of flakes of amorphous PET material, there is a disadvantage of these conventional methods in that the required operating temperatures are also above the softening point of the amorphous PET flakes and these therefore also adhere to the contact body. This in turn reduces the yield of purified PET material, i.e. the conventional methods are very ineffective.